Such a system for the optical transmission of a digital signal over an optical fiber with dispersion at the operating wavelength, with an optical sender at the transmitting end whose optical output is frequency-modulated by the digital signal, and having an optical receiver which converts its optical input into an electrical signal that corresponds to its intensity and recovers the digital signal from the electrical signal is known from: A. R. Chraplyvy et al: "8 Gbit/s FSK Modulation of DFB Lasers with Optical Demodulation", Electronics Letters, Mar. 2, 1989, Vol. 25, No. 5, pages 319 to 321.
When digital signals are transmitted at a high bit rate (in the giga-bit range), the chromatic dispersion (also known as material dispersion) of the beam waveguide at the operating wavelength of the optical transmission system is a problem, insofar as it limits the path length over which a high bit rate digital signal can be transmitted. On the one hand, it is desirable today to have an operating wavelength for the optical transmission in the range of 1550 nm, since suitable fiber-optic amplifiers are available for such wavelengths, and, on the other hand, the use of standard single mode beam waveguides, since these have already been used many times. For that reason, the problem of the chromatic dispersion of the beam waveguide must be solved in a different way than by selecting the operating wavelength or selecting the type of beam waveguide.
The known system describes the following solution: The intensity of the semiconductor laser at the transmitting end of the system is not modulated by the digital signal to be transmitted, as is normally the case, but rather by the frequency of its optical output signal. This modulation is called FSK modulation (FSK=Frequency Shift Keying), whereas the earlier intensity modulation is called "ASK" modulation (ASK=Amplitude Shift Keying). The FSK modulation is attained by modulating the injection current of the semiconductor laser in a clearly weaker form, namely by a clearly smaller increase in modulation than would be the case if the usual ASK modulation was to be obtained.
The frequency modulation gives the transmitted optical signal a smaller spectral range than would be the case with intensity modulation, so that the chromatic dispersion of the beam waveguide no longer has such a detrimental effect.
The known system has an optical interferometer at the receiving end, which converts the frequency modulation of the received optical signal into an amplitude modulation, and an optical receiver for direct reception, which receives the intensity modulated optical signal and recovers the transmitted digital signal from it. An optical receiver for direct reception is usually understood to be an arrangement with an optical detector, a preamplifier, an amplifier and a regenerator (the latter is sometimes also called decision circuit), where the optical detector, together with the preamplifier and the amplifier, converts the time process of the received optical signal's intensity into a corresponding time process of an electrical signal, and the regenerator recovers the digital signal from the electrical signal. For example, such optical receivers are explained in the book "Optical Fibers" by J. Geissler et al, Pergamon Press, Oxford, New York, Toronto, Sydney, Frankfurt, 1986, page 439, or in H. Hamano et al; proc. ECOC '90, Amsterdam, pages 45 to 48.
The known system is more expensive than earlier systems because of its optical interferometer, and therefore disadvantageous from the cost structure point of view, even if it has a higher output.